Question about 1/2 power switch

flatfive · Post by **flatfive** » Fri 05/09/08 11:45 am

I've tried to understand what's being said about this modification. Someone suggested a layout which I'm enclosing. From what I understand, it lifts the ground of two of the cathodes. Is this correct, and are there problems with doing this? Do you have to change the impedance?

Alexo · Post by **Alexo** » Fri 05/09/08 11:50 am

Yes, it lifts the cathodes of one pair of tubes. Rather than disconnect their cathodes from ground, I prefer to run them through a 10K 2 watt resistor to ground, this effectively cuts them off and the trickling current through the 10K cathode resistor keeps the tubes happier.

I would change the impedance, though I think others have reported no problems when they didn't.

flatfive · Post by **flatfive** » Fri 05/09/08 5:04 pm

Can it be a 10k 1/2 watt resistor? That's all I have.

Post by **markh** » Sat 05/10/08 7:53 am

1/2 watt might be a bit light. You could try two 22k or 33k 1/2w in parallel or two 4k7 or 6k8 in series.

I don't worry about impedance at half power.

- The circuit is spec'd for twice the power you're using, so there in no real strain anywhere
- The reason you're doing half power is you *want* it to be quieter. Mis-matching the impedance reduces output power even further.

--mark

creeble · Post by **creeble** » Sat 05/10/08 1:04 pm

I agree with Mark on the impedance; no big deal. Sounds great. My switch just lifts the cathodes (and changes the cathode resistance) -- not exactly sure why having some current flowing is better than none?

Eric.

flatfive · Post by **flatfive** » Sat 05/10/08 1:12 pm

I have 2 5k 2W resistors I could put in series.

Post by **zaphod_phil** » Sat 05/10/08 7:06 pm

creeble wrote:I-- not exactly sure why having some current flowing is better than none?

It helps prevent cathode poisoning, which happens if you run a tube heated but with no voltage across it for very long. There are probably some other good reasons which I can't think of right now....

TDuly · Post by **TDuly** » Sun 05/11/08 12:46 am

zaphod_phil wrote:
creeble wrote:I-- not exactly sure why having some current flowing is better than none?
It helps prevent cathode poisoning, which happens if you run a tube heated but with no voltage across it for very long. There are probably some other good reasons which I can't think of right now....

Well, there are many production amps using the half power cathode ground lift without it so 6 of one and a half dozen of another your mileage may vary.

Alexo · Post by **Alexo** » Sun 05/11/08 3:54 pm

With the price of good tubes where it is now, and where it's likely headed, I like to take all the precautions I can.

flatfive · Post by **flatfive** » Tue 05/13/08 9:52 pm

I'm pretty new to this, so somebody help me with this 10k resistor. Wouldn't I need a separate wire to ground that has the 10k resistor at one end, so that it would trickle when the switch lifts the ground?

flatfive · Post by **flatfive** » Tue 05/13/08 10:44 pm

Maybe just put the resistor across the switch?

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Question about 1/2 power switch

Question about 1/2 power switch